The present invention relates to a method of producing a microlens array, a liquid crystal display device incorporating the microlens array and a method of producing the liquid crystal display device, and a projector using the liquid crystal display device as a light bulb.
Projectors using an LCD (Liquid Crystal Display Device), DMD (Digital Mirror Device), or LCOS (LC·ON·SILICON) as a light bulb has been actively developed. From the viewpoints of function and shape, projectors are classified into a data projector mainly used for monitor display for personal computers, a front projector or a rear projector mainly used for AV for home theaters and the like, and a rear projector for TV. Meanwhile, from the viewpoint of the number of light bulbs, projectors are classified into a one-screen type, a two-screen type, and a three-screen type. The light bulbs are classified into a transmission type and a reflection type.
The higher luminance characteristics of projectors may be required in the future. To meet such a requirement it is primarily expected to improve optics. For example, it is expected to enhance the luminance of a light source to be used, to shorten the arc length (for realizing pointed light source) in the case of using an arc lamp, to optimize optical members, and to miniaturize optical members.
To meet the above requirement, it is secondarily expected to increase the aperture ratio of a light bulb as a key device of a projector. In this case, it is basically required to realize a finer structure and a higher aperture ratio of the device at the pixel level. If liquid crystal is used as an electro-optical medium, however, the aperture ratio of pixels cannot be enhanced only by providing a simple fine structure of the device. The reason for this is as follows: namely, since liquid crystal is a continuous body, a shielding black matrix having an area being large enough to prevent leakage of light from reverse tilt domains and to prevent leakage of light of thin film transistors for driving the liquid crystal must be provided, with a result that the aperture ratio of pixels are correspondingly sacrificed.
To improve the utilization efficiency of light emitted from a light source and also to enhance the luminance, an attempt has been made to mount microlens arrays to liquid crystal display devices. For example, a flat display device incorporating a microlens array has been disclosed in Japanese Patent Laid-open No. Hei 2000-206894. A microlens array incorporated in a high precision liquid crystal display device (liquid crystal panel) for a related art liquid crystal projector has been produced by using a glass substrate such as a quartz substrate or a neoceram glass substrate (hereinafter, the glass substrate used for a microlens array is sometimes referred to as “cover glass”). To be more specific, a method of forming a microlens array using the cover glass by a wet-etching or dry-etching process or a 2P (Photo-Polymerization) process has been put into practical use. In each case, a region in which a microlens array is to be formed is composed of a transparent resin. The thickness of a cover glass for supporting such a transparent resin has been reduced by polishing or grinding in a controlled manner, and a transparent conductive film (for example, ITO film) for a display device has been formed on the cover glass, as needed.
A related art method of producing a microlens array by using a wet etching process will be described with reference to FIGS. 1A to 1D.
In a step shown in FIG. 1A, after a quartz substrate is cleaned, a resist is applied on the quartz substrate, and is patterned into a pattern corresponding to an array pattern of pixels by exposure and development. In a step shown in FIG. 1B, the quartz substrate is subjected to isotropic etching via the resist, to form spherical lens planes R. In addition, a film of a metal, polysilicon, or amorphous silicon excellent in chemical resistance may be used as a mask in place of the resist. The etching may be performed by using an HF or BHF based etchant.
In a step 1C, a cover glass is stuck on the surface of the quartz substrate, and a gap therebetween is filled with a transparent resin having a refractive index different from that of quartz by vacuum injection, spin coating, or spraying. The resin in the spherical lens planes formed by wet etching is perfectly cured by UV irradiation or heating. Examples of the resins used herein include an epoxy based resin, an acrylic based resin, a silicon based resin, and a fluorine based resin, each of which is curable by UV-irradiation or heating. In this way, microlenses arrayed in a pattern corresponding to an array pattern of pixels are formed. Finally, in a step 1D, the cover glass is polished, and a transparent electrode made from ITO is formed, to form a counter substrate. While not shown, the counter substrate is stuck on a drive substrate on which pixel electrodes and thin film transistors are previously formed, and liquid crystal is injected in a gap therebetween, to obtain an active matrix type liquid crystal display device.
FIG. 2 shows a schematic configuration of optics (mainly, illumination optics) of a related art projector. The projector includes a light source 1101, a first microlens array 1102, a second microlens array 1103, a PS synthesizing element 1104, a condenser lens 1105, a field lens 1106, a liquid crystal panel 1107, and a projection lens 1108, which are arranged in this order along an optical axis 1100. The microlens array 1102 has a plurality of microlenses arrayed in a two-dimensional pattern, and the microlens array 1103 has a plurality of microlenses arrayed in a two-dimensional pattern. The PS synthesizing element 1104 includes a plurality of half-wave plates 1104A at positions each of which corresponds to a space between adjacent two of the microlenses of the second microlens array 1103.
In this projector, illumination light emitted from the light source 1101 passes through the microlense arrays 1102 and 1103, to be divided into a plurality of micro-beams. The light emerged from the microlens arrays 1102 and 1103 is made incident on the PS synthesizing element 1104. Light L10 incident on the PS synthesizing element 1104 contains a P-polarized component and an S-polarized component perpendicular to each other within a plane perpendicular to the optical axis 1100. The PS synthesizing element 1104 separates the light L10 incident thereon into two kinds of polarized light components L11 and L12 (P-polarized component and S-polarized component). Of these polarized light components L11 and L12, the polarized light component L11 (for example, P-polarized component) emerges from the PS synthesizing element 1104 with its polarization direction (for example, P-polarization) kept as it is, and the polarized light component L12 (for example, S-polarized component) is converted into the other polarized light component (for example, P-polarized component) by the half-wave plates 1104A, and the converted light component L12 emerges from the PS synthesizing element 1104. As a result, the two separated polarized light components L11 and L12 are directed in a specific direction.
The light emerged from the PS synthesizing element 1104 passes through the condenser lens 1105 and the field lens 1106, and illuminates the liquid crystal panel 1107. The micro-beams divided from the light by the microlens arrays 1102 and 1103 are enlarged at an enlargement ratio determined by the focal distance “fc” of the condenser lens 1105 and the focal distance “f” of the microlenses 1103M of the second microlens array 1103, to illuminate the entire incident plane of the liquid crystal panel 1107. Accordingly, a plurality of the enlarged beams are superimposed on the incident plane of the liquid crystal panel 1107, to realize uniform illumination as a whole. The liquid crystal panel 1107 spatially modulates the incident light on the basis of image signals, and the light emerged from the liquid crystal panel 1107 is projected on a screen (not shown) by the projection lens 1108, to form an image on the screen.
FIG. 3 is a typical perspective view showing one example of a liquid crystal panel. A liquid crystal panel (liquid crystal display device) shown in the figure has a flat panel structure including a pair of substrates 1201 and 1202 and liquid crystal 1203 kept therebetween. A pixel array portion 1204 and a drive circuit portion are integrated on the lower substrate 1201. The drive circuit portion is separated into a vertical drive circuit 1205 and a horizontal drive circuit 1206. Terminals 1207 for external connection are formed on a peripheral upper end of the lower substrate 1201. The terminals 1207 are connected to the vertical drive circuit 1205 and the horizontal drive circuit 1206 via wiring lines 1208. Gate lines G and signal lines S are formed on the pixel array portion 1204. A pixel electrode 1209 and a thin film transistor (TFT) 1210 for driving the pixel electrode 1209 are formed at each of intersections between the gate lines G and the signal lines S. A pixel P is composed of a combination of the pixel electrode 1209 and the thin film transistor 1210. A gate electrode of the thin film transistor 1210 is connected to the corresponding gate line G, a drain resin thereof is connected to the corresponding pixel electrode 1209, and a source region thereof is connected to the corresponding signal line S. The gate line G is connected to the vertical drive circuit 1205, and the signal line S is connected to the horizontal drive circuit 1206. The vertical drive circuit 1205 sequentially selects each pixel P via the gate line G. The horizontal drive circuit 1206 writes an image signal on the selected pixel P via the signal line S. The lower substrate 1201, on which the pixel electrodes and the thin film transistors (TFTs) are integrated, is called as a TFT substrate. While not shown, a counter electrode and color filter are formed on the upper substrate 1202, and therefore, the upper substrate 1202 is called as a counter substrate.
Such a microlens array must meet the requirement toward higher precision as well as the requirement toward higher luminance. For example, as the panel size of a liquid crystal display device becomes small, the pixel size becomes small in proportion thereto, and correspondingly, a cover glass must be made thin. Although a cover glass has been thinned by polishing or grinding, such polishing or grinding has a limitation in thinning the cover glass at a desired accuracy, thereby making it difficult to ensure the uniformity and flatness required for design. If the accuracy and flatness of the plane of a cover glass for a microlens array is insufficient, there arises a problem that mechanical stress may occur at the time of assembling the microlens array in a liquid crystal display device. Also, in the case of thinning a cover glass to 30 μm or less along with the requirement toward higher definition of a panel, there arises another problem that waviness or warping of the cover glass may occur by stress due to shrinkage caused by curing of an optical resin forming the microlens array or a difference in thermal expansion coefficient between the optical resin and the cover glass.
In the case of using the above-described active matrix type liquid crystal display device as a light bulb of a projector, such a liquid crystal display device is more strongly required for higher definition and high luminance. From this viewpoint, a high temperature polysilicon thin film transistor capable of realizing high definition is used as a switching device for driving each pixel. Along with the demand toward a finer switching device, a microlens array is required to have a finer structure. To meet such a requirement, a technique of integrating a microlens array to a substrate of an active matrix type liquid crystal display device has been developed. For example, a method of producing a microlens array incorporating substrate has been disclosed, for example, in Japanese Patent Laid open No. Hei 5-341283, Hei 10-161097, and 2000-147500.
A duel microlens array structure is regarded as an ideal structure capable of realizing the maximum luminance, wherein a microlens array functioning as condenser lenses is assembled in a counter substrate on the light incident side, and a microlens array functioning as field lenses is assembled on a TFT substrate side. Such a duel microlens array is able to enhance the effective aperture ratio of pixels at maximum; however, because of the most difficulty in producing the duel microlens array, any practical production method thereof has been not disclosed at present. It is to be noted that an LCD having a duel microlens array structure is often called as an MTMLCD abbreviated from “Microlens Substrate-TFT Substrate-Microlens Substrate LCD”.